Lytic enzymes such as chitinase and β-1,3-glucanase are known as plant proteins having an antifungal activity against plant pathogenic microorganisms. In vitro experiments have shown that these enzymes can exert the effect if employed alone (Schlumbaum et al. (1986), Nature 324, pp. 365-367), but enhanced effect can generally be obtained if a combination of two or more of such enzymes is used (Mauch et al. (1988), Plant Physiol. 88, pp 936-942). It is known that the growth inhibition concentration of these lytic enzymes against filamentous fungi should be typically about several tens to several hundreds of μg/ml when used alone, or about several μg/ml per enzyme when used in combination. However, none of these lytic enzymes has been reported to have an antifungal effect against rice blast (Magnaporthe grisea), which causes extensive damage to rice crops.
Antifungal peptides (AFP) of low-molecular weight such as defensin also have an antimicrobial activity. Among them, Ca-AMP1 (Japanese Domestic Announcement No. 505048/96), CB-1 (Oita et al. (1996), Biosci. Biotech. Biochem. 60, pp. 481-483), Rs-AFP1 and Rs-AFP2 (Terras et al. 1992, J. Biol. Chem. 267, pp. 15301-15309), and Ace-AMP1 (Japanese Domestic Announcement No. 501424/97) have been reported to have an antifungal effect against rice blast. These low-molecular weight peptides inhibit 50% of the growth of various plant pathogenic microorganisms including the one mentioned above at a concentration in the order of several μg/ml.
Attempts have also been made to create a disease-resistant plant by isolating the gene for a lytic enzyme or a low-molecular weight antifungal peptide and transfecting it into a plant (Broglie et al. (1991), Science 254, pp. 1194-1197; Terras et al. (1995), The Plant Cell 7, pp. 573-588). A recent study of rice reported that transformant rice obtained by overexpressing rice-derived chitinase exerted increased rice blast resistance (Nishizawa et al. (1999) Theor. Appl. Genet. 99:383-390).
Other pathogenic microorganism-resistant plants created by gene introduction have also been reported such as for PR protein (Alexander et al. (1993) Proc. Natl. Acad. Sci. USA 90: pp. 7327-7331), glucose oxidase (Wu et al. (1995) Plant Cell 7: pp. 1357-1368), stilbene synthase (Hain et al. (1993) Nature 361: pp. 153-156), etc.
However, many existing cases fail to obtain transgenic plants having practically acceptable resistance. This may be attributed to the low expression level of the transgenes, and more essentially the low antifungal activity of the antifungal proteins so far reported. Therefore, it would be desirable to identify and practically apply a more potent antifungal protein than conventional ones.